PROJECT SUMMARY/ABSTRACT Extensive knowledge of the genetic mutations responsible for congenital hearing loss and imbalance has led to gene-based therapeutic strategies aimed at rescuing sensory function. The mouse is the dominant model system because of the availability of natural and induced mutations, the accessibility of the neonatal inner ear, and its responsiveness to genetic manipulation. A striking observation from these studies is that virus-mediated gene therapies and pharmacotherapies targeted to the postnatal day 0 (P0) through P5 mouse inner ear yield optimal rescue of hearing and balance. Intervention thereafter dramatically lessens or entirely eliminates therapeutic benefits. Critically, the P0-P5 mouse inner ear is functionally immature with hearing onset at P12 consonant with the emergence of the acoustic startle reflex. In humans, acoustic startle arises at gestational week 19 during the second trimester of pregnancy, suggesting that the window of therapeutic efficacy from P0-P5 in the mouse may predicate a prenatal window of efficacy in the human fetus. The conceptual basis of this proposal is that the early neonatal mouse inner ear functionally models the prenatal human inner ear. To discern if gene therapies defined in the early neonatal mouse inner ear may safely and effectively translate to the clinic, a higher vertebrate model system characterized by the precocious emergence of fetal hearing is needed. Our long-term goal is to establish a rhesus macaque model system to test fetal versus neonatal gene therapy to treat congenital deafness and imbalance. In Aim 1, we will define the onset of fetal hearing in the rhesus macaque. Pure tones at 100, 250, 500, 1000, or 3,000 Hz will be transmitted across the maternal abdomen with increasing intensities. Ultrasonic assessment of acute head, arm, or torso movements will indicate startle. We predict that startle to lower frequency stimuli will emerge first during development as they do in the human fetus. We further hypothesize that the optimal time to intervene therapeutically will precede the age of hearing onset. In Aim 2, we will define a fetal survival surgery to access the inner ear. An adeno-associated viral vector encoding green fluorescent protein (GFP) will be microinjected into membranous labyrinth. The viral transduction efficiency will be estimated by whole mount immunofluorescence to detect GFP. We hypothesize that an AAV2-based vector pseudotyped with a synthetic or naturally occurring capsid will robustly transduce the majority of immature hair cells in the fetal inner ear. In Aim 3, a CRISPR/Cas9-based genome editing technology will be deployed to create rhesus embryos with bi-allelic mutations in harmonin. We hypothesize that correct targeting will produce a model of Usher syndrome type 1C characterized by congenital deafness and profound vestibular dysfunction. Successful completion of the proposed studies will define the optimal gestational age to initiate fetal gene therapy in rhesus; identify an AAV vector capable of delivering harmonin to fetal sensory hair cells; and create a primate model of congenital inner ear disease. These resources will be deployed in future studies to test the safety and efficacy of fetal versus neonatal gene therapy to rescue hearing and balance.